Add support for anonymous nested statics

[carolynzech]

rust-lang/rust#121644 added support for anonymous nested allocations to statics. This PR adds support for such statics to Kani.

The idea is to treat an anonymous GlobalAlloc::Static the same as we would treat a GlobalAlloc::Memory, since an anonymous static is a nested memory allocation. To frame this change in terms of the tests:

pointer_to_const_alloc.rs contains a test for the GlobalAlloc::Memory case, which we could already handle prior to this PR. The MIR looks like:

alloc3 (size: 4, align: 4) {
    2a 00 00 00                                     │ *...
}

alloc1 (static: FOO, size: 16, align: 8) {
    ╾─────alloc3<imm>─────╼ 01 00 00 00 00 00 00 00 │ ╾──────╼........
}

meaning that FOO contains a pointer to the immutable allocation alloc3 (note the alloc3<imm>, imm standing for "immutable").

anon_static.rs tests the code introduced in this PR. The MIR from example_1 looks almost identical:

alloc2 (static: FOO::{constant#0}, size: 4, align: 4) {
    2a 00 00 00                                     │ *...
}

alloc1 (static: FOO, size: 16, align: 8) {
    ╾───────alloc2────────╼ 01 00 00 00 00 00 00 00 │ ╾──────╼........
}

Note, however, that alloc2 is mutable, and is thus an anonymous nested static rather than a constant allocation.
But we can just call codegen_const_allocation anyway, since it ends up checking if the allocation is indeed constant before declaring the global variable in the symbol table:

kani/kani-compiler/src/codegen_cprover_gotoc/codegen/operand.rs

Line 556 in 319040b

alloc.mutability == Mutability::Not && !has_interior_mutabity,

Resolves #3904

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